Stable aqueous impregnating solutions prepared from hydrolyzed alkyltrialkoxysilanes

ABSTRACT

This invention relates to solutions of silanols prepared by hydrolysis of alkyltrialkoxysilanes whose alkoxy groups are partially or completely hydrolyzed. Preferred hydrolyzed alkyltrialkoxysilanes are of the formula CH3-CH2-CH2-Si(OH)n(OH&#39;)3-n.

This is a divisional application of U.S. Ser. No. 304,480 filed Sept.22, 1981, now U.S. Pat. No. 4,352,894.

This invention relates to solutions of silanols prepared by hydrolysisof alkyltrialkoxysilanes whose alkoxy groups are partially or completelyhydrolyzed.

These solutions are stable for as long as several days, depending on theconditions of their preparation. They are particularly well suited foruse as water repellents for inorganic-oxidic materials.

It is known that alkyltrialkoxysilanes can be used as impregnatingagents for masonry, concrete or other inorganic oxidic materials, fromGerman Pat. No. 20 29 446. While these alkyltrialkoxysilanes areliquids, they are employed as dilute solutions, the preferred solventsbeing alcohols. It has not been possible up to now to use solutions inwater since alkyltrialkoxysilanes are but sparingly, if at all, solublein water. Hydrolytic splitting of the ester groups usually also will notgive a stable water-soluble product since under the conditions ofhydrolysis the OH groups of the silanols formed are so reactive thatthey react quickly with other hydroxyl groups which may come either fromthe inorganic-oxidic materials or from vicinal silanols. Because of thepossibilities for such reaction, aqueous solutions ofalkyltrialkoxysilanes are not stable.

On the other hand, the reactivity of the OH groups of alkylsilanolsresults in rapid anchoring of the alkyltrialkoxysilanes on theinorganic-oxidic surface, with which the silanols formed as asintermediate during hydrolysis react to produce a strong bond betweenthe alkylsilane and the surface to be impregnated. However, thisreaction resulting in a strong bond can occur only after the hydrolysisof the alkyltrialkoxysilane has been completed.

Alcoholic solutions of alkyltrialkoxysilanes as impregnating agentsfurther have the drawback that they are flammable. Especially whenworking in closed rooms, the evaporating alcohol can easily lead to afire. The use of water as solvent in place of alcohol with a view toavoiding this drawback has not been possible because of the insolubilityof alkyltrialkoxysilanes in water.

In the case of hydrolytic decomposition in an aqueous medium, however,such solutions are not stable, because of the reactivity of the silanolsthen forming, and quickly become turbid; and this instability of aqueoussolutions of silanols is actually aggravated by the addition of basicmedia as required in many applications. (See U.S. Pat. No. 3,879,206.)

While aqueous solutions of alkyltrialkoxysilanes which are used asimpregnating agents are known from German Pat. No. 10 69 057, theseinvolve special silanes whose ester groups are glycol esters, which aredifficult to hydrolyze and therefore will not form silanols with waterwithout special additives. Thus no silanols are present in thesesolutions. Moreover, these special silanes do not act in the mannerdescribed above through the reaction of free hydroxyl groups with thesurface but through the formation of a silicone resin that forms a filmon the surface.

Thus there has been a need to develop an impregnating solution on thebasis of alkyltrialkoxysilanes in which these silanes are present inhydrolyzed form and which is stable and contains no flammable solvent.

By way of filling this need, stable aqueous solutions ofalkyltrialkoxysilanes have now been discovered in which the alkoxygroups are partially or completely substituted by hydroxyl groups. Thealkyl groups of these silanols are straight-chain and may have up to 4carbon atoms. The preferred alkyl group is the n-propyl group. Thealkoxy groups of the alkyltrialkoxysilanes from which the presentsilanols are prepared have alkyl radicals with from 1 to 4 carbon atoms.The preferred silanols have the formula CH₃ --CH₂ --CH₂ --Si(OH)_(n)(OR')_(3-n), wherein R' stands for an alkyl radical having from 1 to 4carbon atoms and n can assume values between 1 and 3.

Surprisingly, it has now been found that under specific conditions saidtrialkoxysilanes can be hydrolyzed to the corresponding water-solublesilanols, and that the aqueous solution which is so obtained is stable.While it is known that the mixing of alkyltrialkoxysilanes withacidulated water will yield the corresponding silanols, such solutionsare unstable unless special measures are adopted. The silanols claimed,however, will be stable in aqueous solution if they are prepared at roomtemperature and the pH value of the solution is between 1.0 and 7.0, andpreferably between 2.0 and 3.5.

The closer the pH value is to the neutral point in the preparation ofthese solutions, the longer the preparation of stable silanol solutionswill take. It is therefore advisable to not exceed a pH of 5.0 in theirpreparation. The preferred pH range specified will result in preparationtimes that are acceptable in actual practice. The pH value is preferablyadjusted by the use of strong mineral acids such as hydrochloric acid orsulfuric acid. However, strong organic acids such as formic acid arealso suitable for this purpose.

In preparing these solutions, a mixture of alkyltrialkoxysilane andwater is used without heating. The amount of water should be greaterthan the amount necessary for stoichiometric reaction. Advantageously,from 0.5 to 60 weight percent alkyltrialkoxysilane, based on the totalsolution, is used in preparing the solution. It is not necessary to heatthis mixture since silanol formation is exothermic. However, the mixturemay be heated slightly to accelerate the reaction; but temperaturesabove 40° C. should be avoided.

The alcohol liberated during hydrolysis will still be present in thesilanol solution obtained. It is not necessary to eliminate it bydistillation. The silanol solution may be diluted further with water, asrequired for a given application.

The concentration of the alkylsilanols in the solution may range from0.5 to 45 weight percent. Preferably from 2 to 40 weight percentsolutions are used.

For the purposes of the present invention, "stable" means that thesilanols which are in solution will not spontaneously convert tohigher-molecular-weight, water-insoluble siloxanes. In other words,depending on their acidity the solutions will remain free of turbidityfor as long as 80 hours and longer, during which time they caneffectively be used for impregnation. Such solutions standing for anextended period of time may further contain oligomerization products ofthe claimed silanes in solution.

It would have been reasonable to expect that water repellency could beimparted with the novel aqueous solutions of alkylsilanols to a givenmaterial as effectively as with an alcoholic solution ofpropyltrialkoxysilane. Surprisingly, however, it has been found thataqueous solutions of silanols are far better water repellents thanalcoholic solutions of propyltrialkoxysilanes, for example. Thus,practically complete water repellency can be imparted to expandedchivadolimni or trachyte "glass", for example, both of which aresiliceous materials used for insulating purposes, with a just 0.75%solution of silanol in water, whereas a 1% ethanolic solution of thecorresponding alkylsilane will impart only about 40% water repellency.(See Example 2.) In the case of calcareous sandstone, thewater-repelling effect of a 7.5% silanol solution is about equal to thatof a 40% alcoholic propyltrialkoxysilane solution. (See Example 3.)Better water repellency is imparted also to porous clinker brick with a1% aqueous silanol solution than with a 40% alcoholicpropyltrialkoxysilane solution. (See Example 4.)

The materials to which water repellency is to be imparted generally areof an inorganic-oxidic and preferably siliceous nature, and they mayalso be foamed. These materials include synthetic or natural buildingmaterials such as calcareous sandstone, natural stone (sandstone, forexample), trachyte, asbestos, asbestos cement, bloated or expanded clay,the various types of concrete, such as heavy concrete, light-weightconcrete, aerated and foamed concretes, brick, roofing tile, mortar andplaster. These materials may be treated with the new impregnatingsolutions also on site to impart water repellency to them without therisk of solvent fires or toxic off-gases.

However, the water-repelling effect of aqueous silanol solutions is notlimited to said oxide-silicate compounds. A high degree of waterrepellency can be imparted with the new alkylsilanol solutions also tofoamed polyvinyl alcohol plastics. (See Example 6.)

EXAMPLE 1 Preparation of an aqueous silanol solution

50 g propyltrimethoxysilane is mixed with 50 g water to which 3 drops of1% HCl has been added. With stirring at room temperature, the silanehydrolyzes within 10 to 15 minutes with mild evolution of heat, forminga clear solution of theoretically 37 g silantriol. The solution remainsstable for at least three days before the onset of turbidity. It can bediluted with any desired amount of water to obtain the silaneconcentration required for a given end use.

If in place of propyltrimethoxysilane the corresponding ethoxy compoundis used, the situation will be the same. The aqueous solution thentheoretically contains 29 g silantriol.

EXAMPLE 2 Imparting water repellency to expanded chivadolimni

100 g each of the mineral chivadolimni of a particle size ranging from0.5 to 2.5 mm and a specific gravity of 2400 kg/cm³ were intimatelymixed with 200 g each of the water-repellent solutions specified under(b) to (d) below and then dried for 2 hours at 120° C. The material sopretreated was then charged to a height of 24.6 cm into an upright tubewith an inside diameter of 5 cm which had been closed at the bottom witha screen, and which was then weighed. The charge was held at the top bya plunger. Then the tube was immersed for 15 minutes above its fillinglevel in water at room temperature, removed from the water, allowed todrip for 5 minutes while inclined 45 degrees, and weighed. Thedifference in the weight of the charged tube before and after immersionis a measure for the water repellency. The uptake of water is expressedin weight percent, based on the dry weight of the mineral used.

                  TABLE 1                                                         ______________________________________                                        Treating agent      Uptake of water                                           ______________________________________                                        (a)   Untreated         265                                                   (b)   Ethanol           236                                                   (c)   PTMO, 1 wt. % dissolved in                                                                      168                                                         ethanol                                                                 (d)   Propylsilanol, 1 wt. % dis-                                                                      71                                                         solved in water                                                         ______________________________________                                         PTMO = Propyltrimethoxysilane                                            

As a blank test with water-saturated mineral showed, the amount of watermechanically adhering in the charged tube was in each case over 50% andmust be deducted from all values given, so that practically completewater repellency was obtained with (d).

In the case of (c), the measurement was repeated after the silanizedmaterial had been stored for 16 days. The result was the same.

EXAMPLE 3 Impregnation of calcareous sandstone

A calcareous sandstone was dipped for 1 minute in the test solutionslisted in Table 2, whereupon it was left to dry for 7 days at roomtemperature, weighed, and then stored hollow in water, in which it wasimmersed to a level 1 cm above its top surface. The amount of watertaken up by the stone so stored was determined from time to time byweighing.

                  TABLE 2                                                         ______________________________________                                                       Storage time                                                                             Uptake of water,                                    Impregnating solution                                                                        in water   percent                                             ______________________________________                                        (a) Water          1      day   11.5                                                             5      days  11.8                                                             10     days  12.2                                          (b) Aqueous hydrolyzate                                                                          1      day   0.3                                               of 10% PTMO    5      days  1.0                                                              10     days  1.0                                           (c) Alcoholic solution                                                                           1      day   0.4                                               of 40% PTMO    5      days  1.0                                                              10     days  1.1                                           ______________________________________                                         PTMO = Propyltrimethoxysilane                                            

It is apparent from this table that a silanol solution prepared with 10%PTMO has as good a water-repelling effect as a 40% alcoholic solution ofthe corresponding trimethoxysilane.

EXAMPLE 4 Imparting water repellency to clinker brick

Overburned porous clinker bricks were dipped as in Example 3 for 1minute in the test solution specified in Table 3. After drying, theywere weighed. The weighed bricks were immersed in water to a level 1 cmabove their top surface. The uptake of water by the bricks so stored wasdetermined from time to time by weighing.

                  TABLE 3                                                         ______________________________________                                                       Storage time                                                                             Uptake of water,                                    Impregnating solution                                                                        in water   percent                                             ______________________________________                                        (a) Water          1      day   6.5                                                              5      days  7.2                                                              10     days  7.6                                           (b) Aqueous hydrolyzate                                                                          1      day   1.3                                               0.5% PTMO      5      days  1.8                                                              10     days  2.0                                           (c) Aqueous hydrolyzate                                                                          1      day   0.6                                               of 1.0% PTMO   5      days  0.8                                                              10     days  1.0                                           (d) Ethanolic solution                                                                           1      day   0.2                                               of 40% PTMO    5      days  0.4                                                              10     days  1.4                                           ______________________________________                                         PTMO = Propyltrimethoxysilane                                            

Evaluated over a period of 10 days, an aqueous silanol solution of 1%PTMO is more effective than a 40% alcoholic silane solution.

EXAMPLE 5 Imparting water repellency to spheres of bloated clay

The bloated-clay spheres which are used as fillers for concrete have adiameter ranging from 4 to 8 mm and are porous and highly absorptive. Torender them water-repellent, samples of such spheres were intimatelymixed with an impregnating solution corresponding to 10% of the weightof the spheres. This amount of solution was absorbed completely by thespheres. The latter were then dried for 2 hours at 110° C. and aftercooling stored in weighed lots under water. The uptake of water by thespheres was determined from time to time.

                  TABLE 4                                                         ______________________________________                                                       Storage time                                                                             Uptake of water,                                    Impregnating solution                                                                        in water   percent                                             ______________________________________                                        (a) Water          1      day   18                                                               3      days  22                                                               6      days  27                                            (b) Aqueous hydrolyzate                                                                          1      day   4                                                 of 0.1% PTMO   3      days  6                                                                6      days  11                                            (c) Aqueous hydrolyzate                                                                          1      day   3                                                 of 2.0% PTMO   3      days  4                                                                6      days  8                                             (d) Aqueous hydrolyzate                                                                          1      day   1                                                 of 4.0% PTMO   3      days  2                                                                6      days  4                                             (e) Ethanolic solution                                                                           1      day   4                                                 of 0.5% PTMO   3      days  10                                                               6      days  15                                            (f) Ethanolic solution                                                                           1      day   6                                                 of 0.5% IBTMO  3      days  12                                                               6      days  17                                            (g) Ethanolic solution                                                                           1      day   6                                                 of 4.0% IBTMO  3      days  10                                                               6      days  13                                            ______________________________________                                         PTMO = Propyltrimethoxysilane                                                 IBTMO = Isobutyltrimethoxysilane                                         

The percentages representing the active-substance content given in thetable are based on the weight of the spheres.

It is apparent from the table that the effect of an aqueous silanolsolution of 0.1% PTMO is not duplicated by solutions of silane inalcohol of from 5 to 40 times higher concentration.

EXAMPLE 6 Imparting water repellency to a foamed plastic

A sheet of foamed polyvinyl acetate plastic 5 mm thick was cut intostrips measuring 5×17 cm. When such a strip, weighing about 14 g, wasimmersed for 15 minutes in cold water, it took up an amount of watercorresponding to about 200% of its dry weight. This process was found tobe reversible.

However, when the water contained a hydrolyzate of 2.5% PTMO, theprocess proved irreversible. After drying, the uptake of water by thefoamed-plastic strip after 15 minutes' immersion then was only 15%, andafter 30 minutes' immersion, 19%. A comparative test with an ethanolicsolution of 2.5% isobutyltrimethoxysilane showed that after impregnationwith the water repellent the uptake of water ranged from 80 to 93%.

It will be appreciated that the instant specification and examples areset forth by way of illustration and not of limitation, and that variouschanges and modifications may be made without departing from the spiritand scope of the present invention.

What is claimed is:
 1. Stable aqueous solution of hydrolyzedalkyltrialkoxysilanes wherein the solution has a pH value from 2 to 7.2. Stable aqueous solution as claimed in claim 1, wherein the hydrolyzedalkyltrialkoxysilane has the formula CH₃ --CH₂ --CH₂ --Si(OH)_(n)(OR')_(3-n), wherein R' stands for an alkyl radical having from 1 to 4carbon atoms and n can assume values between 1 and
 3. 3. Stable aqueoussolution as claims in claim 1, wherein said hydrolyzedalkyltrialkoxysilanes constitute from 0.5 to 45 weight percent of thesolution.
 4. Stable aqueous solution as claimed in claim 3, wherein saidhydrolyzed alkyltrialkoxysilanes constitute from 2 to 40 weight percentof the solution.
 5. The stable aqueous solution of hydrolyzedalkyltrialkoxysilanes of claim 1, wherein the alkoxy group has 1 to 4carbon atoms.